Many optical systems contain powered or unpowered mirrors. A mirror reflects a primary beam of incident light at an angle equal to the angle of incidence. In the ideal case where there is a single perfectly collimated incident primary beam, the reflected beam contains only light from that incident primary beam. However, in many cases there are stray light rays reflected from the mirror at other angles. The stray light rays may result from a lack of perfect collimation of the incident primary beam, reflections from surfaces within the optical system such as lens coatings or supports, or from other light sources. Where the primary beam is in the infrared, the stray light may result from the radiation of the internal structure of the optical system.
The stray light rays may have several adverse effects. When they reach a detector, the stray light rays reduce the signal-to-noise ratio of the optical system by increasing the background signal or noise. The signal resolution of the optical system may be compromised. There also may be damage to sensitive optical components that are not designed for compatibility with the stray light.
The usual approach to avoiding stray light effects is the use of mechanical masks such as apertures, stops, and/or light baffles. These mechanical devices are positioned along the incident or reflected primary light paths to block the stray light rays. The mechanical masks are effective, but they lead to an increase in the size of the optical system because they are spaced apart from the mirror. The mechanical masks are therefore one of the impediments in attempts to miniaturize optical systems. Additionally, the mechanical masks limit the size of the useful area of the mirror. The smaller the opening of the mechanical mask through which the incident beam or reflected beam passes, the less area of the mirror is available for use and the less is its light-handling capacity.
There is a need for an improved approach to avoiding stray-light effects in optical systems that employ mirrors. The present invention fulfills this need, and further provides related advantages.